In patients with coronary artery disease at risk for sudden cardiac death, malignant ventricular tachycardia (VT) is treated with a combination of implantable defibrillators and antiarrhythmic drugs. While effective in preventing sudden cardiac death, this strategy is plagued with medication side-effects and toxicities, and device related iatrogenic complications. The ideal treatment for VT would be eradication of the arrhythmia. This can be accomplished with greater than 90% efficacy through surgical resection of the infarcted arrhythmogenic tissue; however, this is an open surgical procedure associated with significant morbidity and mortality. Radiofrequency (RF) catheter ablation is an effective less-invasive alternative, but is largely limited to hemodynamically-tolerated VT-which accounts for less than 10% of all VT. However, the approach to catheter ablation of VT is undergoing a paradigm shift. Instead of trying to precisely identify the critical portions of the VT circuit during tachycardia, "substrate mapping" is performed in sinus rhythm to identify the arrhythmogenic tissue. Then, a probabilistic approach to catheter ablation is undertaken using strategically-placed linear RF lesions. While this strategy has proven to be successful in small non-randomized studies, the safest and most efficacious method to deliver the RF ablation is not known. Also, the role of substrate ablation in the primary prevention of ICD shocks is unknown. We now propose to conduct prospective randomized clinical trials: a) comparing the standard non-irrigated to the saline-irrigated RF ablation catheter for substrate-mapping and ablation in the treatment of patients with multiple ICD shocks (the THERMO COOL IDE trial), and b) examining the role of substrate-mapping and RF ablation in the primary prevention of ICD shocks (the SMASH-VT trial). Furthermore, the abilitv of a new 3-dimensional mapping system to rapidly identify the arrhythmogenic substrate will be examined in a porcine model of healed myocardial infarction with inducible VT. Finally, using this advanced mapping system to identify the substrate, the effect of catheter-mediated cryoablation will be compared to RF ablation using a saline-irrigated catheter on the inducibility of VT in this animal model system.